The present invention relates to a capillary electrophoresis system and method. Specifically, the invention relates to a capillary electrophoresis system and method that provides multiple simultaneous analysis of a sample by positioning first ends of a plurality of capillaries in a single container such that the plurality of capillaries provide simultaneous electrophoretic analysis for a sample in the container.
Electrophoresis is a well-known technique for separating small amount of macromolecules. Increasingly, electrophoresis has become an indispensable tool for the biotechnology and other industries and is used extensively in a variety of applications, including the separation, identification and preparation of samples of nucleic acids, proteins and carbohydrates. Of increasing interest in the broader field of electrophoresis is capillary electrophoresis (CE), where particular entities of species are moved through a medium in an electrophoretic channel of capillary dimensions under the influence of an applied electric field. In some applications, the medium in the capillary electrophoretic channel is a buffer. Oftentimes, however, the medium is a gel which acts as a sieving matrix to help retard and separate the individual molecules as they migrate through the capillary channel.
Capillary electrophoresis is typically performed using fused silica capillary tubes. The tubes may have inner channel diameters in the range of about 20-1000 micro-meters or microns (xcexcm). Capillary electrophoresis is generally employed to analyze an extremely small quantity of samples, such as proteins or nucleic acids. Other benefits of CE include rapid run time and high separation efficiency. A capillary electrophoresis system or apparatus usually charges a glass capillary having an inner diameter of not more than 100 xcexcm with a migration medium such as buffer or gel, introduces a sample into an end of the capillary, and applies a high voltage across the capillary to separate molecules based on differences in charge-to-size ratio. Since capillaries have large surface area relative to their small volume, resulting in high cooling efficiency, high voltages can be applied in analyzing small quantities of samples at high speed and in high resolution.
To illustrate a capillary electrophoresis process, one may take the example of DNA sequencing using gel capillary electrophoresis. Prior to the electrophoresis analysis, the DNA sample is prepared, using well-known methods, so that a solution of DNA fragments of all possible lengths corresponding to the same total sequential order is obtained, with each fragment terminated with a tag label corresponding to the identity of the given terminal base.
The separation process employs a capillary tube filled with conductive gel. To introduce the sample, one end of the capillary channel is placed into the DNA reaction vial. After a small amount of sample enters the capillary end, both capillary ends are then placed in separate buffer solutions. A voltage potential is then applied across the capillary tube. The voltage drop causes the DNA sample to migrate from one end of the capillary to the other. Differences in the migration rates of the DNA fragments cause the sample to separate into bands of similar-length fragments. As the bands traverse the capillary channel, the bands are typically read at some point along the channel using one of several detection techniques.
Usually, multiple DNA preparation reactions are performed in a commercially available micro-titre tray having many separate low-volume wells, each holding on the order of 200-1000 micro-liters (xcexcL). The micro-titre trays come in standard sizes. In the biotech industry, the currently preferred micro-titre tray has a rectangular array comprising 8 rows of 12 columns of wells. The centers of adjacent wells found in a single row are separated by approximately 0.9 cm, although this figure may vary by one or two tenths of a millimeter. The same holds for the spacing between adjacent wells in a single column. The rectangular array of 96 wells has a footprint within an area less than 7.5 cmxc3x9711 cm.
Miniaturization has allowed more wells to be accommodated in a single micro-titre tray having the same footprint. New trays having four times the density of wells within the same footprint have already been introduced and are fast becoming the industry standard. Thus, these new trays have 16 rows and 24 columns with an adjacent well spacing of approximately 0.45 cm.
Various attempts have been made to perform multiple capillary electrophoresis simultaneously. For example, U.S. Pat. No. 6,027,627 to Li et al., the contents of which are incorporated by reference, discloses an automatic electrophoretic system which employs a capillary cartridge having a plurality of capillary tubes. The cartridge has a first array of capillary ends projecting from one side of a plate. The first array of capillary ends are spaced apart in substantially the same manner as the wells of a micro-titer tray of standard size. This allows one to simultaneously perform capillary electrophoresis on samples present in each of the wells of the tray.
U.S. Pat. No. 6,048,444 to Takahashi et al., the contents of which are incorporated by reference, discloses a capillary electrophoresis apparatus having a plurality of capillaries which are filled with migration medium and have first ends into which samples are injected and second ends from which components included in the samples are eluted. The apparatus further comprises a sheath flow cell in which the second ends are arranged in a straight line at first predetermined intervals and are terminated; means for flowing a buffer solution in the sheath flow cell from the lower part of the sheath flow cell to the upper part of the sheath flow cell; and means for detecting a component eluted from each of the capillaries in the sheath cell near the second ends.
U.S. Pat. No. 6,054,032 to Haddad et al., the contents of which are incorporated by reference, discloses a capillary electrophoresis array that includes a plurality of capillary tubes arranged adjacent each other in a generally longitudinal orientation with each of the tubes having an inlet end, an outlet end, and internal diameter no greater than about 100 microns; and a registration assembly in which adjacent tubes are held in place with a fixed lateral spacing relative to each other at both the inlet and the outlet ends of the tubes. The array is flexible along the length of the tubes.
Despite these progresses, the efficiency and reliability of capillary electrophoresis is still in need of improvement in various aspects. One particular problem is that, in many cases, and often for unknown reason, a run of capillary electrophoresis for a sample would simply fail, resulting in the waste of time and materials. Although this problem could be alleviated somewhat by running multiple analysis for the sample, in many situations, there is simply not enough sample, or the samples are too expensive, to do so using available prior art methods. Furthermore, in other situations, more electrophoresis data for a sample is desirable to achieve reliable results.
The invention provides a capillary electrophoresis system for providing simultaneous multiple analysis of a sample. The system comprises: at least one assembly of capillaries comprising a plurality of capillaries having first and second capillary ends and being held in a spaced relationship relative to one another at a point proximate to the first capillary ends, a cross-section of the first capillary ends defining a minimum bounding polygon having a footprint with a major dimension; and a container comprising at least one compartment having an opening and a bottom, and configured to hold a single body of liquid; wherein the assembly is positioned such that its first ends extend through the opening and into the at least one compartment to thereby simultaneously contact a single body of liquid, when said liquid is contained therein.
In one aspect of the above capillary electrophoresis system, the first ends of the plurality of capillaries in the capillary assembly do not contact each other. Preferably, the first ends of the plurality of capillaries are substantially parallel to each other.
In another aspect, the capillary assembly comprises two capillaries, wherein the minimum bounding polygon defined by the cross-sections of the two capillaries is a rectangle. Alternatively, the capillary assembly comprises three capillaries, wherein the minimum bounding polygon defined by the cross-sections of the three capillaries a triangle.
In yet another aspect of the capillary electrophoresis system of the present invention, the container is a one dimensional micro-titer strip and the at least one compartment is a well of said micro-titer strip. Alternatively, the container is a micro-titer tray comprising a two-dimensional array of wells, and the at least one compartment is one of said wells. Further, the system may comprise a plurality of capillary assemblies, with each assembly being positioned such that the first capillary ends of the plurality of capillaries in each assembly are inserted into a corresponding one of wells in either the one dimensional micro-titer strip or the two dimensional micro-titer array. According to an aspect of the invention, the centers of the wells are spaced apart by a distance greater than the major dimension of the cross-section of the first capillary ends in an assembly. Preferably, the major dimension is less than 4000 xcexcm.
The capillary electrophoresis system of the present invention may further comprise an electrode that simultaneously contacts the single body of liquid together with the first ends of the plurality of capillaries of the at least one assembly. Also the system may comprise detection means to simultaneously detect the electrophoresis process in the plurality of capillaries.
In another aspect, the present invention provides an assembly of capillaries which comprises: a plurality of capillaries having first and second capillary ends and being held in a spaced relationship relative to one another at a point proximate to the first capillary ends, wherein a cross-section of the first capillary ends defines a minimum bounding polygon having a footprint with a major dimension of less than about 4000 xcexcm.
In one embodiment of the capillary assembly, the major dimension of the minimum bounding polygon is less than about 3000 xcexcm. In another embodiment, the major dimension of the minimum bounding polygon is less than about 2000 xcexcm and, in yet another embodiment, the major dimension of the minimum bounding polygon is less than about 1000 xcexcm.
In a preferred embodiment of the above capillary assembly, the capillary assembly further comprises an electrode, which is in the form of a metal wire, and that the portion of the plurality of capillaries and the metal wire are configured such that the cross-section of the plurality of capillaries and the metal wire together define the minimum bounding polygon.
In a third aspect, the present invention provides a capillary electrophoresis system for providing simultaneous multiple analysis of a sample. The system comprises: at least one assembly of capillaries comprising a plurality of capillaries having first and second capillary ends and being held in a spaced relationship relative to one another at a point proximate to the first capillary ends with a cross-section of the first capillary ends defining a minimum bounding polygon having a footprint with a major dimension; and a container comprising at least one compartment having an opening and a bottom, and configured to hold a single body of liquid; wherein the at least one assembly is positioned such that the first capillary ends extend through the opening and into the at least one compartment with each tip of said first capillary ends terminating at a height no greater than a height h above the bottom when a volume of the compartment below the height h is less than about 500 xcexcL.
In one embodiment of this capillary electrophoresis system, the volume below the height h in the compartment is less than about 200 xcexcL. In another embodiment, the volume is less than about 10 xcexcL. In yet another embodiment, the volume is less than about 1 xcexcL.
The present invention further provides a method of capillary electrophoresis for providing simultaneous multiple analysis of a sample. The method comprises: providing a container having at least one compartment having an opening and a bottom, and configured to contain a sample; inserting first ends of a plurality of capillaries that have first and second ends through the opening and into a sample contained in the compartment such that the first ends contact the sample; applying a voltage potential difference between the first and second ends to thereby cause the sample to migrate from the first ends towards the second ends within each of the plurality of capillaries; and detecting migrated portions of the sample in each of the plurality of capillaries.
In one aspect of the present invention""s method, the container is a one dimensional micro-titer strip or a two dimensional micro-titer tray and the at least one compartment is a well of said micro-titer strip or tray.
In another aspect of the method, the volume of the sample is less than about 200 xcexcL. Alternatively, the volume of the sample is less than about 10 xcexcL or less than about 1 xcexcL.
In yet another aspect, the present invention""s method further comprises providing an electrode that simultaneously contacts the sample together with the ends of the first ends of the plurality of the capillaries.